It is generally desirable with rechargeable alkaline manganese dioxide-zinc cells for them to be zinc-limited. That means that the capacity of the cathode and the capacity of the anode are chosen such that the zinc is completely used up before the useful reversible portion of the manganese dioxide capacity is exhausted. If manganese dioxide is discharged beyond the reversible capacity, its characteristic of being rechargeable would, to all intents and purposes, be destroyed. The issue of rechargeable alkaline manganese, and the pre-reduction of the material for the cathodes, are discussed in a U.S. Patent assigned to the same assignee as this application, namely U.S. Pat. No. 5,011,752, issued Apr. 30, 1991.
As is well known, a rechargeable manganese-zinc cell has the basic structure of a cathode which is comprised substantially of manganese dioxide, an anode having zinc particles included in its composition, an aqueous alkaline electrolyte, a current collector in intimate physical contact with the anode, a container for the cell, and a separator which physically is located between the anode and the cathode and which electrically insulates them from each other. The separator is at least partially wettable by the electrolyte; and in keeping with the present invention, at least one layer of the separator is an ion permeable membrane which is otherwise unpenetrable by particles such as zinc dendrites.
In keeping with a particular aspect of the present invention, the anode is prepared in such a way that it contains zinc particles, zinc oxide, and an amount of finely divided precipitated copper--metallic or elemental copper--which is in a manner deposited on or between the zinc particles in such a way that it establishes an electrically conductive, low resistance structure within the structure of the anode. Put in other words, the metallic or elemental copper provides an interconnecting metallic structure which has a sponge-like appearance and characteristic, but which is otherwise structurally integral. This feature is important to the life of the cell, especially its shelf-life over a prolonged storage, whether the cell is primary or secondary in nature.
However, particularly in the case of a secondary cell, the metallic copper structure within the zinc node remains in place after the zinc metal powder has been used up during discharge of the cell, and it remains in place for redeposition of new zinc during the charge cycle which will follow.
On the other hand, if the cell is overcharged, the copper acts as a catalyst--indeed, a very active catalyst--which is in intimate contact with the recharged zinc. Thus, the matter of the oxygen recombination cycle which occurs locally around and within the anode structure is catalytically driven to recombine the oxygen without excessive oxygen gas evolution away from the anode.
It should also be noted that if the cell is extensively discharged or over-discharged, the finely divided metallic or elemental copper can be oxidized to become copper oxide, thereby preventing oxygen gassing if the cell reaches cell reversal.